Cause

Ubuntu 18.04 comes with PostgreSQL 10 where support for unencrypted passwords was removed. Plesk does not support it currently and this is registered as Plesk bug #PPPM-8775 that is planned to be fixed in future product updates.

As I can see from the screenshot, PostgreSQL 10.4 is installed on the server. I have updated the article with a little bit more details regarding this issue.

In short, the error is actual for PostgreSQL 10 on any OS, Ubuntu 18.04 just has it out-of-the-box.

I suggest removing PostgreSQL 10 from the server via Plesk installer and with the help of `apt purge postgresql*` command. Then, disable custom repositories and install PostgreSQL 9 that comes for Ubuntu 16.04 out of the box.

@Alexandr Redikultsev, using Ubuntu 18.04, should I install PostgreSQL 9 via Plesk or via console? Will an upgrade to PostgreSQL 10 be possible (as soon as the Plesk bug #PPPM-8775 has been fixed), without any data loss?

Right now it is hard to say whether or not PostgreSQL upgrade will be supported or not, however considering the fact that the bug is already fixed in development (Plesk Onyx 17.9), I can suggest installing testing version of Plesk on some fresh non-production server, installing PostgreSQL 9 and then performing an upgrade to check how it goes. Here is the link on how to install testing version of Plesk (not for production use):

Hi @Alexandr Redikultsev, thanks for your reply. As you said, the bug is already fixed in development. So chances are good that we get the bug fix applied to our instances pretty soon? However, wouldn't it be more sensible to install PostgreSQL 10 and set the password via console? PostgreSQL should work properly, just without the comfortableness of Plesk, right? And then just wait for the bug fix in order to be able to manage it via Plesk from that moment on? Thank you very much!

Currently, there is no ETA regarding the backport of the fix, so I cannot promise that it will be applied soon, but this article will be updated once it will be fixed for Plesk Onyx 17.8.

As for your action plan, you understand it correctly, the only thing that is broken is integration with Plesk, overall PostgreSQL should work fine.

And most probably, once the bug will be fixed, you will be able to reset the password in the way that Plesk will accept it. No promises that it will be easy or one-click, but I think it should be possible :)

I have just setup a fresh server with Plesk Onyx 17.8 ... cannot manage any PostgreSQL databases from the UI... as a result migrations from old servers don't work either. This Bug is actually quite critical, is there still no ETA?

It is not recommended to upgrade production environments to preview versions (now it is 17.9).

Preview versions are only for introductory test use. Technical issues of preview versions are not investigated by Plesk Technical Support and upgrade from Preview to stable Plesk version is also not officially supported.